Best Hardware for Apple Motion?

So my colleagues and I are currently making a short with using motion using a top of the line macbook pros, and everything has has been going relatively well, but we have one issue that keeps coming up regarding render times. We're working with some pretty complex graphics and sometimes I have issues with the amount of time required to render sequences in Best, sometimes projects crash or renders fail and sometimes renders suggest that they'd hundreds of hours to complete. So I'm considering getting a mac pro to render our projects while we continue to work on other scenes on the macbooks.

I have three questions before I make any purchase though:

1) What is the best video card I can get for Motion?
2) If I have the top of the line video card, how important is it that I get a newer model of mac pro? Would it have significantly improved performance from a cheaper older model with the same amount of RAM?
3) What would the performance differential be in making this leap in the first place? Would we see dramatic decreases in render times mac pro compare to another top of the line macbook pro?